From a quick search, I found this thread:
https://g80.bimmerpost.com/forums/sh....php?t=1788420
Seems to be a good start to the discussion.
Many (most?) of us running snow tire set-ups for winter, are running the front 19x9.5 (with 275/35 19 tires) all around.
I suspect that the more G8x cars get tracked, the more we'll start seeing square 19 (and even square 18) set-ups for track duty.
I've also seen some folks running square 20s, and the 21/20 staggered set-up you mention above. The bigger rims look great, although I happen to think a chunky sidewall (35 or even 40 aspect ratio) looks good too, with the right wheel.